Not very familiar with this subject was just interested. I would think @itonewbie  would know this subject from a-z. found this from IRS:

Social Security Number

If your spouse is a nonresident alien and you file a joint or separate return, your spouse must have either a Social Security Number (SSN) or an Individual Taxpayer Identification Number (ITIN). To get an SSN for your spouse, apply at a social security office or U.S. consulate. You must complete Form SS-5. You must also provide original or certified copies of documents to verify your spouse's age, identity, and citizenship. If your spouse is not eligible to get an SSN, he or she can file Form W-7 with the IRS to apply for an ITIN. Refer to Taxpayer Identification Numbers (TIN) for more information.

Here is link: https://www.irs.gov/individuals/international-taxpayers/nonresident-alien-spouse